This is a website that I worked on five years ago, and I haven't done anything to it since. Does anyone have any ideas why this error message has just started to appear? http://www.mwisilo.com/index.php

The actual error message is on the last line. So what it means is that your database server is not allowing access to the username and IP address listed there. So something has been changed somewhere. You'll want to confirm your MySQL credentials haven't changed and manually updated them in /data/settings.dat.php